1. Field of the Invention
The present invention relates generally to an elongated jacketed low pressure discharge lamp, such as a fluorescent lamp, and, more particularly, is concerned with a one-piece spacer end cap for each end of the elongated jacketed discharge lamp.
2. Description of the Prior Art
The light output of any fluorescent lamp depends on the mercury vapor pressure inside the lamp. Optimum pressure for maximum light output for most fluorescent lamps occurs when the coolest spot on the lamp envelope tube is about 100.degree.F. One important factor affecting lamp temperature is ambient temperature and wind draft conditions. As the lamp temperature is raised above 100.degree.F., the mercury vapor pressure within the elongated discharge envelope tube of the lamp increases and light output and lamp wattage slowly drop. At lower lamp envelope tube temperatures, the mercury condenses on the tube, pressure drops, and light output falls sharply and wattage drops slowly. This effect of temperature on mercury vapor pressure and thereby on light output is common to all fluorescent lamp designs.
Fluorescent lamps are widely used in both indoors and outdoors applications. Problems typically encountered when using fluorescent lamps outdoors fall into two categories: starting and operating. Fluorescent lamps become harder to start when cold. Once started, successful operation depends on how hot the lamp becomes. Thus, for outdoor use, fluorescent lamps are commonly jacketed, employing an elongated outer light-transmitting glass sleeve disposed over the discharge envelope tube, to reduce heat loss from the tube and protect the tube from wind drafts.
Also, jacketed fluorescent lamps typically have end cap assemblies to shield and protect the ends of the lamp discharge envelope tube against cold wind drafts. Representative of the end cap assemblies used in the prior art are the ones disclosed in U.S. patents issued to Shanks (U.S. Pat. No. 3,358,167) and Hammer (U.S. Pat. No. 3,453,470) and assigned to the same assignee as the present invention. Each of these prior art end cap assemblies include an annular spacer of insulating material and an annular cup-shaped shell of thin sheet metal material. The annular insulating spacer fits on one end of the discharge envelope tube and has a narrow annular groove which receives and seats an end of the outer glass sleeve. The annular metal shell has an annular skirt or sidewall which is force-fitted about and grips the periphery of the annular insulating spacer to put the spacer, sleeve and envelope tube in compression to assure a rigid assembly. The shell also has a face or end wall which then overlies a base on the end of the discharge envelope tube sealing off any empty space. The end wall has a central aperture through which protrudes a contact housing projecting from the base of the tube end.
Overall, these lamp end cap assemblies have functioned satisfactorily in the low temperature environments in which they are designed to be used. However, it has been perceived by the inventor herein that it would be desirable to devise additional improvements in order to make it easier to seat the outer sleeve end in the annular insulating spacer groove and to reduce the number of parts and the amount of labor required to install the parts while retaining the wind draft shielding capability of the end cap.